Defibrillators are used to supply a large amount of electrical energy to a patient through paddles. Prior to delivery, this energy is accumulated on a high-voltage capacitor. The amount of energy supplied to the patient is a predetermined value, selected as appropriate for the patient.
Defibrillators typically measure the voltage across the high-voltage capacitor and calculate the energy accumulated on it according to the following equation, where E is energy, C is the capacitance of the capacitor, and V is the voltage across the capacitor: EQU E=1/2CV.sup.2. (1)
Ideally, in determining the appropriate voltage for a desired energy, the capacitor's actual capacitance is used rather than its nominal value because any variance between in the capacitance value used and its actual value results in a corresponding error in the amount of energy delivered to the patient. For example, if the actual capacitance is five percent less than the value used to determine voltage, then the energy supplied to the patient will also be five percent less than intended.
A capacitor's actual capacitance can differ from its nominal value for many reasons. The difference could be due to manufacturing variations. However, such variations could be detected and accounted for during manufacture of the defibrillator.
Just as important are changes in the capacitance which occur after the defibrillator leaves the factory. For example, some capacitors lose capacitance as they age. Also, high-voltage metalized film capacitors or paper capacitors can lose capacitance when charged to voltages near their maximum rated voltages, a situation common in defibrillators. The dielectric material separating the metal layers can break down, allowing a localized arc. Although the dielectric material is destroyed at this location by the arc, the metalization at the location also evaporates from the heat of the arc, thereby maintaining the operational integrity of the capacitor. This type of capacitor is known as a self healing capacitor. However, this small reduction in capacitor plate area decreases the capacitance by a corresponding amount. This effect is cumulative over the life of the capacitor, and may decrease the capacitance by more than five percent.
For the foregoing reasons, there is a need for a defibrillator which can determine the actual capacitance of its storage capacitor, and store the value for future energy calculations. To adequately satisfy this need, the defibrillator should be able to calibrate its capacitor as needed throughout the life of the defibrillator.